This invention relates to a latex additive for wellbore drilling fluids. More specifically, it pertains to an additive for reducing the loss of drilling fluid into the formations surrounding the wellbore.
For the production of hydrocarbon wells, boreholes are drilled into subterranean formations. Following standard procedures, a fluid is circulated during drilling from the surface through the interior of the drill string and the annulus between drill string and formation. The drilling fluid, also referred to as xe2x80x9cdrilling mudxe2x80x9d, is used to accomplish a number of interrelated functions. These functions are:
(1) The fluid must suspend and transport solid particles to the surface for screening out and disposal;
(2) It must transport a clay or other substance capable of adhering to and coating the uncased borehole surface, both (a) to exclude unwanted fluids which may be encountered, such as brines, thereby preventing them from mixing with and degrading the rheological profile of the drilling mud, as well as (b) to prevent the loss of downhole pressure from fluid loss should the borehole traverse an interval of porous formation material;
(3) It must keep suspended an additive weighting agent (to increase specific gravity of the mud), generally barites (a barium sulfate ore, ground to a fine particular size), so that the entire column of drilling fluid is not interrupted upon encountering pressurized pockets of combustible gas, which otherwise would tend to reduce downhole pressure, as well as creating a xe2x80x9cblowoutxe2x80x9d in which the fluid and even the drill stem are violently ejected from the well, with resulting catastrophic damages, particularly from fires;
(4) It must constantly lubricate the drill bit so as to promote drilling efficiency and retard bit wear.
The industry distinguishes between largely three classes of drilling fluids: oil-based, water-based and so-called synthetic muds. Whereas oil-based muds are recognized for their superior qualities for most of the drilling operations themselves, they become increasing undesirable due to their impact on the environment and stricter environmental legislation. Water-based muds are expected to replace oil-based mud as the drilling fluid of choice in major geographical areas.
A drilling fluid typically contains a number of additives. Those additives impart desired properties to the fluid, such as viscosity or density. One class of additives is used as fluid loss agents to prevent the drilling fluid from entering into porous formations.
The basic mechanism of fluid loss control is generally the formation of a filter cake at the interface of the porous or permeable formation layers. As part of the drilling fluid is forced into the formation by the higher pressure within the wellbore, larger particles and additives are left behind and accumulate at the face of the formation. The filter cake thus formed can be regarded as a membrane that protects the formation from further invasion of wellbore fluids. Fluid-loss control agents are selected in view of their quality to form a competent filter cake.
Known examples of such fluid-loss control agents are water-soluble polymeric additives added to the drilling fluid to improve the sealing of the filter cake. These fluid-loss polymers are most commonly modified celluloses, starches, or other polysaccharide derivatives and are subject to temperature limitations. In particular, most start to fail around 105-120 degrees C.
Latices on the other hand are described for example in the U.S. Pat. No. 5,770,760 using latex to thicken water-based drilling fluids. The latex is added to the mud and chemically treated to produce the functional polymer that is in a solubilized form.
The use of latices for the purpose of fluid loss control is described for example in the U.S. Pat. Nos. 4,600,515 and 4,385,155. In those applications, however, polymer latices are used in a water-soluble form.
It is therefore an object of the present invention to provide a novel class of fluid loss agents for drilling fluids.
The invention comprises the use of polymer latices for fluid loss control in water-based drilling fluids. The latices used are water insoluble. Preferably, the latices are essentially non-swelling in an aqueous solution.
The latices are selected from known latices such that they are absorbed within a filter cake building up at the interface between the wellbore and porous formations in essentially the same state as they are in the aqueous drilling fluid. Hence the latices used for this application are not coagulated or further crosslinked.
Another selection criterion for suitable latices is that the Tg, or glass transition temperature of the polymer must be lower than the temperature of the drilling application so that the polymer is in a rubbery or fluid state. In this state the polymer particles are deformable which improves the sealing characteristics of the filter cake.
The polymer latices can be of any water insoluble polymers, copolymers or terpolymers, for example synthesized by emulsion polymerization. The main chemical types can be summarized as:
Polymers and copolymers in which the principal repeat units are derived from monoolefinically-unsaturated monomers such as vinyl acetate, vinyl esters of other fatty acids, esters of acrylic and methacrylic acids, acrylonitrile, styrene, vinyl chloride, vinylidene chloride, tetrafluoroethylene and related monomers.
Polymers and copolymers in which the major proportion of the repeat units are derived from 1,3-dienes such as 1,3-butadiene (butadiene) 2-methyl-1,3-butadiene (isoprene) and 2-chloro-1,3-butadiene (chloroprene), with smaller proportions of the repeat units being derived from the monoolefinically unsaturated monomers such as styrene and acrylonitrile, or others of category 1.
Other polymers such as polyisobutenes containing minor amounts of copolymerised isoprene, polyurethanes and other monomer units.
Latices used for the purpose of the present invention include but are not restricted to styrene-butadiene copolymer latex (SBR), and styrene-acrylate-methacrylate terpolymer latex (SA).
Compatibility with other solids present in the drilling fluids may require the use of an additional stabilizer as additive to the water based drilling fluid. This may be the case for certain types of SBR latices. SA latices appear stable at ambient and moderate temperatures (to ca. 60C) but become destabilized at elevated temperatures. Other latex chemistries may be more stable. The stabilizer is generally added at a dosage of 10% of the latex concentration or less. Care must be taken in selection to minimize formation damage from free stabilizer. The most effective stabilizers are anionic surfactants typified by sodium docdecyl sulphate (SDS), Aerosol OT (AOT), and polymeric stabilizers/surfactant such as NPE (a 30% aqueous solution of ammonium salt of sulfated ethoxylated nonylphenols). Nonionic surfactants such as the Triton series, an octylphenol polyether alcohol with varying numbers of ether linkages per molecule, commercially available from Union Carbide. Synperonics can also be used to stabilize the latex.
Further additives as known in the art may be added to impart other desired properties to the mud system. Such known additives include viscosifying agents, filtrate reducing agents, and weight adjusting agents. Other preferred additives are shale-swelling inhibitors, such as salts, glycol-, silicate- or phosphate-based agents, or any combination thereof.